Recently active-matrix organic light-emitting diode (AMOLED) displays with amorphous silicon (a-Si), poly-silicon, organic, or other driving backplane have become more attractive due to advantages over active matrix liquid crystal displays. An AMOLED display using a-Si backplanes, for example, has the advantages that include low temperature fabrication that broadens the use of different substrates and makes flexible displays feasible, and its low cost fabrication that yields high resolution displays with a wide viewing angle.
The AMOLED display includes an array of rows and columns of pixels, each having an organic light-emitting diode (OLED) and backplane electronics arranged in the array of rows and columns. Since the OLED is a current driven device, the pixel circuit of the AMOLED should be capable of providing an accurate and constant drive current.
FIG. 1 shows a pixel circuit as disclosed in U.S. Pat. No. 5,748,160. The pixel circuit of FIG. 1 includes an OLED 10, a driving thin film transistor (TFT) 11, a switch TFT 13, and a storage capacitor 14. The drain terminal of the driving TFT 11 is connected to the OLED 10. The gate terminal of the driving TFT 11 is connected to a column line 12 through the switch TFT 13. The storage capacitor 14, which is connected between the gate terminal of the driving TFT 11 and the ground, is used to maintain the voltage at the gate terminal of the driving TFT 11 when the pixel circuit is disconnected from the column line 12. The current through the OLED 10 strongly depends on the characteristic parameters of the driving TFT 11. Since the characteristic parameters of the driving TFT 11, in particular the threshold voltage under bias stress, vary by time, and such changes may differ from pixel to pixel, the induced image distortion may be unacceptably high.
U.S. Pat. No. 6,229,508 discloses a voltage-programmed pixel circuit which provides, to an OLED, a current independent of the threshold voltage of a driving TFT. In this pixel, the gate-source voltage of the driving TFT is composed of a programming voltage and the threshold voltage of the driving TFT. A drawback of U.S. Pat. No. 6,229,508 is that the pixel circuit requires extra transistors, and is complex, which results in a reduced yield, reduced pixel aperture, and reduced lifetime for the display.
Another method to make a pixel circuit less sensitive to a shift in the threshold voltage of the driving transistor is to use current programmed pixel circuits, such as pixel circuits disclosed in U.S. Pat. No. 6,734,636. In the conventional current programmed pixel circuits, the gate-source voltage of the driving TFT is self-adjusted based on the current that flows through it in the next frame, so that the OLED current is less dependent on the current-voltage characteristics of the driving TFT. A drawback of the current-programmed pixel circuit is that an overhead associated with low programming current levels arises from the column line charging time due to the large line capacitance.